1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method of transmitting a PLCP frame in a WLAN system and a wireless apparatus supporting the method.
2. Related Art
With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among them, a WLAN is a technique which enables wireless access to the Internet at homes or companies or in specific service providing areas through mobile terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), on the basis of radio frequency technology.
Since Institute of Electrical and Electronics Engineers (IEEE) 802 (i.e., the standard organization of WLAN technology) was set up on February, 1980, lots of standardization tasks are being performed.
The initial WLAN technology was able to support the bit rate of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication by using a 2.4 GHz frequency band in accordance with IEEE 802.11, but the recent WLAN technology can support a maximum bit rate of 54 Mbps by using Orthogonal Frequency Division Multiplex (OFDM). In addition, in the IEEE 802.11, the standardization of various techniques, such as the improvements of Quality of Service (QoS), the compatibility of Access Point (AP) protocols, security enhancement, radio resource measurement, wireless access vehicular environment for vehicle environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, is put to practical use or being developed.
Furthermore, as a technique for overcoming limits to the communication speed considered as vulnerabilities in the WLAN, there is IEEE 802.11n which has recently been standardized. The object of the IEEE 802.11n is to increase the speed and reliability of a network and to expand the coverage of a wireless network. More particularly, the IEEE 802.11n is based on a Multiple Inputs and Multiple Outputs (MIMO) technique using multiple antennas on both sides of a transmitter and a receiver in order to support a High Throughput (HT) having a data processing speed of 540 Mbps or higher, minimize transmission errors, and optimize the data rate. Further, the IEEE 802.11n may use not only a coding method of transmitting several redundant copies in order to increase data reliability, but also an Orthogonal Frequency Division Multiplex (OFDM) method in order to increase the data rate.
In addition to a PLCP format supporting legacy STAs, an HT green field PLCP format (that is, a PLCP format efficiently designed for HT STAs) which can be used in a system composed of HT STAs supporting IEEE 802.11n has been introduced into the IEEE 802.11n HT (High Throughput) WLAN system. Furthermore, the IEEE 802.11n HT (High Throughput) WLAN system supports an HT mixed PLCP format which is a PLCP format designed to support an HT system in a system in which legacy STAs and HT STAs coexist.
In the HT mixed PLCP frame, an HT-SIG field is subjected to encoding and interleaving processes and then mapped for modulation. Here, a QBPSK constellation is used. The QBPSK constellation is a constellation shifted from a BPSK constellation by 90°. An HT-SIG field can be simply detected because an L-SIG field uses a common BPSK constellation.
For detailed information about the HT green field PLCP format and the HT mixed PLCP format, reference can be made to “IEEE P802.11n™/D11.0, Draft STANDARD for Information Technology-Telecommunications and Information Exchange Between Systems-Local and Metropolitan Area Networks-Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Amendment 5: Enhancements for Higher Throughput, Clause 20. High Throughput PHY specification” disclosed on June, 2009.
In IEEE 802.11n, 8 bits for CRC check are allocated to the HT-SIG field, thereby being capable of protecting 0-33 bits (0-23 bits are an HT-SIG1 field and 0-9 bits are an HT-SIG2 field) from among 48 bits. In the CRC operation, after a shift register is set to an initial value, input bits are sequentially calculated through the shift register, and the last bit enters the shift register. After all operations are finished, bits remaining in the shift register are obtained as outputs. For example, assuming that (m0 . . . m33)={1 1 1 1 0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0}, CRC bits {c7 . . . c0}={1 0 1 0 1 0 0 0}.
When an HT STA detects the HT-SIG field of an HT mixed PLCP frame, two kinds of operations are possible in addition to a mode in which the HT-SIG field is normally read and operated. The HT STA can be operated in the legacy mode because it has recognized that the HT-SIG field is not the HT-SIG field or, although it has recognized that the HT-SIG field is the HT-SIG field, can inform CRC error through PHY-RXEDN.indication (Format Violation) without transmitting PHY-RXSTART.indication because of errors detected as the result of the CRC execution. At this time, the HT PHY terminal maintains PHY-CCA.indication (BUSY, channel-list) until a received level drops less than a specific CCA sensitivity level (e.g., an energy detection threshold) indicating an idle channel.
In each of the OFDM symbols of a 20 MHz channel of IEEE 802.11n, four subcarriers are composed of a pilot signal. This is for coherent detection robust to frequency offset and phase noise. The pilot signal can be modulated into a BPSK constellation, placed in indices −21, −7, 7, 21, and represented by {0, 0, . . . , 0, 1, 0, . . . , 0, 1, 0, . . . , 0, 1, 0, . . . , 0, −1, 0, . . . , 0}. Meanwhile, the pilot subcarriers are scrambled by a sequence Pn.
With the WLAN being widely spread and applications using the WLAN becoming diverse, a need for a new WLAN system capable of supporting a higher throughput than the data processing speed supported by the IEEE 802.11n is recently gathering strength. A Very High Throughput (VHT) WLAN system is one of the IEEE 802.11 WLAN systems which have recently been proposed in order to support a data processing speed of 1 Gbps or higher.
In IEEE 802.11 TGac in which the standardization of a VHT WLAN system is being carried out, in order to provide the throughput of 1 Gbps or higher, research is being done on a scheme using 8×8 MIMO and a channel bandwidth of 80 MHz or higher and a PLCP format for efficiently supporting each STA in a WLAN system in which a legacy STA, an HT STA, and a VHT STA coexist. As part of improving the performance of an SU-MIMO mode introduced in IEEE 802.11n and an MU-MIMO mode to be newly introduced into the VHT WLAN system, a method of configuring a PLCP frame format capable of effectively supporting the SU-MIMO mode and the MU-MIMO mode and guaranteeing the coexistence by preventing the malfunction of the legacy STA and the HT STA, and a wireless apparatus supporting the method need to be taken into consideration.